Prime Minister Theresa May said late Tuesday that the terror threat level in Britain has been increased from “severe” to “critical” after review by the independent organisation the Joint Terrorism Analysis Center.

The increased threat level means “not only that an attack remains highly likely, but that a further attack may be imminent,” May said. She added “the work undertaken today has revealed that it is a possibility we cannot ignore that there is a wider group of individuals linked to this attack.”

The move came the day after 22 people were left dead after a suicide bombing in Manchester at an Ariana Grande concert. At least 59 others were injured.

Police identified the attacker Tuesday as 22-year-old Salman Abedi, who police said died in the attack. May suggested Abedi could have been part of a bigger network.

May said the change in the threat level means armed military personnel will be deployed to guard “key locations” in Britain, as well as certain events like “sports matches” and “concerts.” She said those personnel will take the place of armed police officers, who will be deployed elsewhere. The armed military personnel will be under the command of local police forces, who will decide how to deploy them. Additional resources will be made available to police as they work to keep Britain safe, May said.

May finished her remarks saying she did not “want the public to feel unduly alarmed,” but encouraged everyone to “stay vigilant.”